Fretting, which is one of the most serious problems that engineers have to deal with in the designing and building of new machines or apparatus, is a particular type of corrosion found on the contact areas of loaded metal surfaces subject to oscillatory or vibratory motion. The problem of fretting occurs not only in the moving parts of an apparatus, but also in the stationary parts therein. Examples of such non-moving parts in which fretting has been observed include shrink fits, press fits, bolted flanges and keyed gears. Examples of such moving parts in which fretting has been observed include bearings undergoing an oscillatory motion, flexible couplings, and reciprocating cams.
Two or more surfaces when rubbed together generate therein a reciprocating motion which, as current belief holds, causes mechanical wear and, consequently, fretting. This reciprocating motion, which is linear, oscillatory or vibratory in nature, is of limited amplitude, producing no more than just a relative slip between the surfaces.
Between contacting metal surfaces, plastic deformation, that is, the interlocking on a microscopic scale of the contacting high points, is thought to cause fretting. A tangential force exerted on the surface does not separate the contacting high points along their original interface. Instead, a small piece of metal which becomes lodged between the contacting metal surfaces is sheared off from the points of contact. Acting as an abrasive, the loose metal particles soon start to wear down the contacting metal.
Once started, fretting corrosion is difficult to stop for the formation of one wear particle inevitably leads to the production of others. Fretting damage can lead to fatigue failures, galling, jamming of critical clearances with abrasive debris, and other undesirable effects. Often times, the damage done by fretting is serious enough to completely immobilize the moving part, or to wear down or corrode a non-moving part.
Of special concern to design engineers is the damage done by fretting corrosion, a type of fretting which occurs between contacting metal (steel, iron, etc.) parts. Oxygen reacts with the metal surfaces, or with the wear particles therefrom, and the corrosion caused by the formed rust can spread and immobilize the entire apparatus.
One solution to the fretting problem is to separate and keep apart contacting surfaces with a lubricant such as a grease composition. To perform satisfactorily, this grease must inhibit fretting corrosion and must also be able to perform at high temperatures.
Greases employed heretofore have generally been found to be unsatisfactory in both preventing fretting or fretting corrosion and in working in a high temperature environment. A new grease composition has been discovered which substantially mitigates or overcomes the fretting and fretting corrosion problems noted above.
It is the object of the instant invention to provide a novel grease composition characterized by having substantial anti-fretting and anti-fretting corrosion properties.
It is another object of the invention to provide an anti-fretting grease composition which is effective in a high temperature environment.